Yamaha TT 600

THE four-stroke off road market has always been limited to a certain segment of eccentrics. Two-strokes, by virtue of their inherent design characteristics, perform and handle better — and are often cheaper. Logically, the two-stroke should be the serious offroad rider's choice.

In fact up to now, the big bore four-stroke has had very little going for it except the fact that it is, well... a four-stroke. It makes a satisfying noise and somehow reminds us of those tales about Velo-cettes and Manx Nortons being the only real bikes ever built on God's Earth.

A four-stroke, like a slice of quiche, has always been to some men, A Real Man's Motorcycle. As a dirt bike, not always very practical, hardly ever the quickest, but nevertheless a proper bike.

The XT 500 and its more competition-orientated brother, the TT 500, were proper bikes, lusty four-strokes with a following that ranged from Durban potheads to desert adventurers. But on the bottom line, when it came to competition use, nobody in their right minds would enter an old XT or TT in an enduro. At least not in standard form — they were too slow, much too heavy and handled like pigs.

In contrast Yamaha's 1983 TT 600 is something of a revelation. It replaces its predecessor so completely and with so much flair, that one wonders whether there wasn't an intermediate model that we missed out somewhere along the line.

The new TT 600 is a street scrambler in the sense that it is sold in a form that barely covers most municipalities' and provincial authorities' requirements for street-legal machines. What I am saying is that it is conceivable that you could ride it to work in the mornings and not get hassled by a man in khaki.

But it is far more than a street scrambler because it's very evident that this bike is not a compromise. Let's look at it: the TT 600 engine is simply an enlarged version of the TT 500 engine which made its first appearance in this country last year, and soon gained a reputation for being a fast, reliable street scrambler.

Bore and stroke are now 95 x 84 mm to give a displacement of 595 cm^{3. The inherent vibration of the large capacity, big bore single is damped out by a contra-rotating balance shaft geared directly to the crankshaft and mounted just behind it. A single overhead camshaft is driven from the crank via a Hy-Vo chain with an automatic tensioner. This operates four valves.}

Like Honda, Yamaha have gone the four-valve route for four-stroke dirt bikes because this system allows a greater port area without af-fecting combustion chamber shape, and the valve springs can individually deal with less reciprocating mass — making the mill more reliable at peak revs.

Ignition is maintenance-free capacitor discharge with a small 12-volt battery to deal with the street legal items.

The fuel-feed system, like the XT 500, consists of the Yamaha patented Duo Intake setup with a twin barrel carburettor (1 mm larger than theXT550's).

The bike has an aluminium kick-start, and starting is reasonably easy for a single of more than half litre displacement — mainly because of the cable-operated, automatic decompression facility which comes into operation when the engine is kicked over.

Like the old big Yam singles, this one has a dry sump lubrication, but the TT 600 doesn't carry its oil in its spine: a new oil tank has been positioned above the swingarm pivot, and this gets the centre of gravity as low as possible.

The bike's running gear is more of an indication of its no-compromise character than the engine. The frame is made from high-tensile steel  and the swingarm is extruded aluminium.

The rear-end consists of the new rising rate Yamaha Monocross linkage suspension, with the single shock positioned low and leaning slightly forward from the end of the swingarm. Compression damping and preload are adjustable, and the setup has a remote reservoir.

The front-end is really out of the top draw: 43 mm Showa fork tubes with aluminium sliders and 300 mm of travel! The front brake, although not a disc, is an exceptionally effective twin-leading shoe stopper.

Overall finish is good. The TT 600 is offered almost in IT enduro spec. It has a plastic fuel tank, tool bag with an impressive set of tools, quick-release axles with snailcam adjusters, side-pull throttle, alloy rims with strong spokes and an O-ring chain. Tyres are 3.00 21 and 150/80 18 IRCs which work well.

It is one of those dirt bikes that feels right, from the time you get on, to when you start takin skills), it's a bike that can be ridden easily by both the novice dirt rider and a veteran expert.

The TT is first and foremost a big bore four-stroke, and this is, perhaps it's most endearing quality. But the fact that for the first time we are offered a four-stroke with such a big engine along with such superb chassis and suspension components, puts this bike into a class all of its own.

For a big bike, it turns fairly easily and although it's not the best for really tight terrains, the wide, wide power band makes riding in most situations a giggle. This sucker has a Kalahari Desert-full of torque: 4,7 kg/m at 5 500 revs/min! It likes lugging over large boulders and that forgiving engine doesn't falter up the steepest of inclines.

Lots has been written about four-stroke power, but in reality a big two-stroke will go up a hill better than a four-stroke. However, I think that the TT 600 could be in with the large strokers when it comes to hills.

However, it's over open territory that the new TT really comes into its own. The quality suspension soaks up high speed bumps and the monolithic powerband can be used to slide the bike safely and predictably through fast turns. It's also got the brakes to match its top-end performance.

This bike's a winner in my opinion one of the best all-round dirtbikes on the market at the moment, and definitely one of those that are the most fun to ride on your favourite trail!

source BIKE & TRACK 1983

To deliver all the power and torque, undiluted to the ground, the TT600 is equipped with the latest Monocross rear suspension, taken directly from our YZ motocross racers.

The monoshock unit is fully adjustable for rebound damping and pre-load. It's also smaller, lighter, more compact and sits lower in the frame. This, combined with a lower, more

centralised oil tank, contributes to an exceptionally low centre of gravity and an appreciable improvement in handling.

Of course, this remarkable assembliage of unequalled 4-stroke power, state-of-the-art chassis and YZ suspension wouldn't be complete without a front end to match. In this case, air-adjustable forks with huge 43mm tubes and no less than 300mm of travel.

Add to all that some of the competition-proven features of our IT enduro racers— quick-release axles, snail-cam chain adjusters, all ADR lighting and instrumentation requirements, tubular engine guards and a very generous tool bag—and what you've got is the first 4-stroke dirt machine that's more than equal to the rigors of enduro, cross-country, desert and 4-stroke motocross racing. Or the pleasures of a weekend of backWoods biking.

Or both.

The TT600's motocross-proven Monocross rear suspension is a true "rising-rate" suspension. Which means that as the rear wheel begins to move upward, the suspension begins to stiffen. The farther the wheel travels, the stiffer both spring rate and shock damping become. The redesigned pivot and strut system allow the reservoired gas/oil shock absorber to sit lower, lowering the centre of gravity.

The TT600's powerplant is one of a kind. It not only has four valves and a dual exhaust system, it has a dual intake system, too.

To take full advantage of the four-valve set-up, our Yamaha Duo Intake System actually utilises two separate carburettors, a primary slide-valve unit and a secondary butterfly-valve unit. That makes for a combined intake area 20% greater than that of a standard single carb.